Effect of straw amendment modes on soil organic carbon,nitrogen sequestration and crop yield on the North‐Central Plain of China

We quantified the effects of different straw return modes on soil organic carbon (SOC), total nitrogen content (TN) and C:N ratios in a wheat/maize double‐cropping agricultural system by analysing their content in different soil aggregate sizes and density fractions under four modes of straw return: (a) no return/retention of wheat and maize straw (Control); (b) retention of long wheat stubble only (Wheat Stubble); (c) retention of long wheat stubble and return of chopped maize straw (Mixed); and (d) return of chopped wheat and maize straw (Both Chopped). The Mixed and Both Chopped straw return modes produced the highest crop yields. Relative to the Control, SOC stock was 9.6% greater with the Mixed treatment and 14.5% greater with the Both Chopped treatment, whereas the Wheat Stubble treatment had no effect on SOC. Mixed and Both Chopped significantly enhanced TN stock relative to the Wheat Stubble and Control treatments. Compared with the Control, the Mixed and Both Chopped treatments increased the mass proportions of large macroaggregates and reduced the silt plus clay fraction; Mixed and Both Chopped caused a significant increase in SOC and TN in large and small macroaggregates; the Mixed treatment significantly increased SOC content in the coarse and fine intra‐aggregate particulate organic matter (iPOM) density fractions of large macroaggregates, whereas Both Chopped increased SOC in the coarse iPOM, fine iPOM and mineral‐associated organic matter (mSOM) density fractions of both large and small macroaggregates; and Mixed and Both Chopped enhanced TN content in coarse iPOM and fine iPOM within small macroaggregates. Although the Mixed treatment was slightly less effective at improving C sequestration in agricultural fields than the Both Chopped treatment, the Mixed treatment may nonetheless be the optimal plant residue management mode in terms of minimizing time and labour due to its ability to improve soil structure, maintain organic carbon levels and provide a means of sustainable crop production in intensive wheat/maize double‐cropping systems.     

Organic carbon in soils of Germany: Status quo and the need for new data to evaluate potentials and trends of soil carbon sequestration

Uncertainties in estimates of soil carbon (C) stocks and sequestration result from major gaps in knowledge of C storage in soils, land‐use history, the variability of field measurements, and different analytical approaches applied. In addition, there is a lack of long‐term datasets from relevant land‐use systems. As in many European countries, a national database on soil organic carbon (SOC) including all relevant information for the determination of soil C stocks is likewise missing in Germany. In this paper, we summarize and evaluate the present state of knowledge on organic‐C contents/pools in soils of Germany and discuss the need for the acquisition and access to new data on soil organic carbon. Despite the number of agricultural sites under permanent soil monitoring, regional surveys on SOC, comprehensive ecosystem studies, and long‐term field experiments, there is a striking lack of data in Germany particularly with regard to agricultural soils. Apart from a missing standardization of methods and homogeneous baseline values, the implementation of a periodic, nation‐wide soil inventory on agricultural soils is required in order to simultaneously record information on land use, land‐use change, and agricultural practice. In contrast, the existing national inventory of forest soils provides information on C‐stock changes in forest soils, although there is some concern with regard to the representativeness of the sampling design to adequately address the problem of spatial heterogeneity and temporal variability. It is concluded that the lack of comprehensiveness, completeness, actuality, data harmonization, and standardized sampling procedures will further prevent the establishment of a SOC database in Germany with regard to the monitoring of trends in soil C pools and fluxes and the assessment of long‐term C‐sequestration potentials of soils under different land use. A future soil inventory should represent the heterogeneity of organic matter through functionally different SOC pools, topsoil characteristics as well as content, pool, and flux data for the deeper mineral‐soil compartments.     

Gaseous carbon emission in relation to soil carbon fractions and microbial diversities as affected by organic amendments in tropical rice soil

The interactive effects of moisture and organic amendments (farmyard manure (FYM), crop residue (CR) and green manure (GM) (Sesbania aculeata) on gaseous carbon (C) emission, soil labile C fractions, enzymatic activities and microbial diversity in tropical, flooded rice soil were investigated. The amendments were applied on equal C basis in two moisture regimes, that is, aerobic and submergence conditions. The CO₂ production was significantly higher by 22% in aerobic than in submergence condition; on the contrary, the CH₄ production was 27% higher under submergence condition. The labile C fractions were significantly higher in GM by 26% under aerobic and 30% under submergence conditions, respectively, than control (without any kind of fertilizer or amendments). Eubacterial diversity identified by PCR-DGGE method (polymerase chain reaction coupled with denaturant gradient gel electrophoresis) was higher under GM followed by FYM, CR, and control and it is pronounced in submerged condition. GM favored the labile C accumulation and biological activities under both submergence and aerobic conditions, which makes it most active for soil–plant interactions compared to other organic amendments. Considering environmental sustainability, the use of GM is the better adoptable option, which could enhance labile C pools, microbial diversities in soil and keep soil biologically more active.     

Land Use and Soil Organic Carbon in China’s Village Landscapes

Village landscapes, which integrate small-scale agriculture with housing, forestry, and a host of other land use practices, cover more than 2 million square kilometers across China. Village lands tend to be managed at very fine spatial scales (≤ 30 m), with managers both adapting their practices to existing variation in soils and terrain (e.g., fertile plains vs. infertile slopes) and also altering soil fertility and even terrain by terracing, irrigation, fertilizing, and other land use practices. Relationships between fine-scale land management patterns and soil organic carbon (SOC) in the top 30 cm of village soils were studied by sampling soils within fine-scale landscape features using a regionally weighted landscape sampling design across five environmentally distinct sites in China. SOC stocks across China’s village regions (5 Pg C in the top 30 cm of 2 × 10 6 km 2 ) represent roughly 4% of the total SOC stocks in global croplands. Although macroclimate varied from temperate to tropical in this study, SOC density did not vary significantly with climate, though it was negatively correlated with regional mean elevation. The highest SOC densities within landscapes were found in agricultural lands, especially paddy, the lowest SOC densities were found in nonproductive lands, and forest lands tended toward moderate SOC densities. Due to the high SOC densities of agricultural lands and their predominance in village landscapes, most village SOC was found in agricultural land, except in the tropical hilly region, where forestry accounted for about 45% of the SOC stocks. A surprisingly large portion of village SOC was associated with built structures and with the disturbed lands surrounding these structures, ranging from 18% in the North China Plain to about 9% in the tropical hilly region. These results confirmed that local land use practices, combined with local and regional variation in terrain, were associated with most of the SOC variation within and across China’s village landscapes and may be an important cause of regional variation in SOC.     

The sensitivity of water extractable soil organic carbon fractions to land use in three soil types

Soil organic carbon (SOC) has a high impact on the sustainability of ecosystems, global environmental processes, soil quality and agriculture. Long-term tillage usually leads to SOC depletion. The purpose of this study was to determine the impact of different land uses on water extractable organic carbon (WEOC) fractions and to evaluate the interaction between the WEOC fractions and other soil properties. Using an extraction procedure at 20°C and 80°C, two fractions were obtained: a cold water extractable organic carbon (CWEOC) and a hot water extractable organic carbon (HWEOC). The results suggest that there is a significant impact from different land uses on WEOC. A lower relative contribution of WEOC in SOC and a lower concentration of labile WEOC fractions are contained in arable soil compared to forestlands. Chernozem soil was characterized by a lower relative contribution of WEOC to the SOC and thus higher SOC stability in contrast to Solonetz and Vertisol soils. Both CWEOC and HWEOC are highly associated with SOC in the silt and clay fraction (<53 µm) and were slightly associated with SOC in the macroaggregate classes. The WEOC fractions were highly and positively correlated with the SOC and mean weight diameter.     

Influence of peach (Prunus persica Batsch) phenological stage on the short-term changes in oxidizable and labile pools of soil organic carbon and activities of carbon-cycle enzymes in the North-Western Himalayas

The labile organic carbon(C) and C-related enzymes are sensitive indicators capturing alterations of soil organic matter(SOM),even in a short-time scale.Although the effects of crop husbandry and land use change on these attributes have been well studied,there is no consensus about how plant phenology may impact them.This study aimed to determine the short-term effect of six distinct phenological stages(PS-1:full bloom;PS-2:fruit set;PS-3:pit hardening;PS-4:physiological maturity;PS-5:60 d after physiological maturity;and PS-6:fall) of peach on the changes in soil organic carbon(SOC) fractions of different oxidizability,labile C pools,and C-cycle enzyme activities in soils,for two consecutive years(2015 and 2016) in the North-Western Himalayas(NWH).Peach rhizosphere soils were sampled at the topsoil(0–15 cm) and subsoil(16–30 cm) layers,along with rhizosphere soils from adjacent perennial grasses,which served as a control.Values for most of the assessed parameters,including very labile C,labile C,microbial biomass C,permanganate oxidizable C,dissolved organic C,mineralizable C,amylase activity,and carboxymethyl-cellulase activity,were significantly(P ≤ 0.05) higher at PS-3 than at other phenological stages of peach.Conversely,a sudden decline in these soil variables was recorded at PS-5,followed by a slight buildup at PS-6,particularly in the topsoil of the peach orchard.Short-term changes in organic C fractions of different oxidizability,influenced by peach phenological stage,significantly(P ≤ 0.05) affected C management index,C pool index,and lability index.Both the C management index and lability index showed their highest values at PS-3 and their lowest values at PS-5,clearly indicating short-term accretion and depletion of SOC,in tandem with the peach phenological events.Principal component analysis suggested that a composite of soil indicators,including microbial biomass C,dissolved organic C,amylase,and invertase,could help detect short-term changes in SOC content.It is concluded that peach phenological events had a major impact on the short-term variations of the studied soil variables,which could be attributed to changes in the above-and belowground plant residues,as well as the extent of nutrients and water acquisition.     

Lability of soil organic carbon in tropical soils with different clay minerals

Soil organic carbon (SOC) storage and turnover is influenced by interactions between organic matter and the mineral soil fraction. However, the influence of clay content and type on SOC turnover rates remains unclear, particularly in tropical soils under natural vegetation. We examined the lability of SOC in tropical soils with contrasting clay mineralogy (kaolinite, smectite, allophane and Al-rich chlorite). Soil was sampled from A horizons at six sites in humid tropical areas of Ghana, Malaysian Borneo and the Solomon Islands and separated into fractions above and below 250 μm by wet sieving. Basal soil respiration rates were determined from bulk soils and soil fractions. Substrate induced respiration rates were determined from soil fractions. SOC lability was significantly influenced by clay mineralogy, but not by clay content when compared across contrasting clay minerals. The lability of SOC was lowest in the allophanic and chloritic soil, higher in the kaolinitic soils and highest in the smectitic soil. Our results contrast with conventional concepts of the greater capacity of smectite than of kaolinite to stabilize SOC. Contents of dithionite-citrate-bicarbonate extractable Fe and Al were inversely related to SOC lability when compared across soil types. A stronger inverse correlation between content of ammonium-oxalate extractable Fe and SOC lability was found when considering the kaolinitic soils only and we conclude that the content of active Fe (hydr-) oxides controls SOC stabilization in the kaolinitic soils. Our results suggest that the validity of predictive models of SOC turnover in tropical soils would be improved by the inclusion of soil types and contents of Fe and Al (hydr-) oxides.     

Thermal oxidation does not fractionate soil organic carbon with differing biological stabilities

Thermal analysis techniques have been used to differentiate soil organic carbon (SOC) pools with differing thermal stability. A correlation between thermal and biological stability has been indicated in some studies, while others reported inconsistent relationships. Despite these controversial findings and no standardized method, several recently published studies used thermal analysis techniques to determine the biological stability and quality of SOC in mineral soils. This study examined whether thermal oxidation at temperature levels between 200°C and 400°C, combined with evolving gas analysis and isotope ratio mass spectrometry, is capable of identifying SOC pools with differing biological stability in mineral soils. Soil samples from three sites being under Miscanthus (C₄‐plant) cultivation for more than 17 years following former agricultural cropland (only C₃‐plant) cultivation were used. Due to natural shifts in ¹³C content, young and labile Miscanthus‐derived SOC could be distinguished from stable and old C₃‐plant‐derived SOC. The proportion of Miscanthus‐derived SOC increased significantly with increasing temperatures up to 350°C in bulk soil samples, indicating increasing oxidation of labile and young SOC with increasing temperatures. Use of density fractions to validate the thermally oxidized SOC from bulk soil samples revealed that the thermal oxidation patterns did not reflect the biological stability of SOC. The suggested biologically labile particulate organic carbon (light fraction from density fractionation) was clearly enriched in Miscanthus‐derived young SOC. The thermal oxidation patterns, however, revealed preferential oxidation of these biologically labile fractions not at low temperatures, but rather at higher temperatures. The reverse was found for the biologically stable mineral‐associated density fraction (heavy fraction). Based on different soil types, it was concluded that the thermal stability of SOC between 200°C and 400°C is not a suitable indicator of the biological stability of SOC and, thus, thermal oxidation is not capable of fractionating SOC pools with differing biological stability.     

长期施肥对棕壤有机碳组分的影响

对起始于1979年的棕壤长期肥料定位试验田2005年的耕层土壤不同有机碳组分进行了测定与分析,以探讨长期施肥影响土壤有机碳的过程及机理。结果显示:长期单施化肥降低了土壤的游离态颗粒有机碳（FPOM-C）含量,但进一步稳定了矿物结合态有机碳(MOM-C),最终提高了土壤总有机碳(TOC)含量;长期施用有机肥和有机肥配施化肥使土壤的FPOM-C、闭蓄态颗粒有机碳(OPOC)、MOM-C以及含量均显著提高,且增加效果好于单施化肥。从各组分有机碳所占比例或相对比值来看,长期施用有机肥和有机肥配施化肥提高了POM-C/TOC比例而降低了MOM-C/TOC比例,使FPOM-C/OPOM-C比值显著增大。表明土壤有机碳结构分组的应用有助于揭示长期施肥影响土壤有机碳的机理。     

水稻-小麦轮作系统中土壤活性有机碳库对长期施用化肥，秸秆和粪便的敏感度

Labile soil organic carbon(SOC) pools, estimated through chemical fractionation techniques, are considered sensitive indicators of management-induced changes in quality and composition of soil organic matter. Although the impacts of organic manure and crop residue applications on C sequestration in rice-wheat system are fairly well documented, their influence on labile SOC pools is relatively less known. Impacts of organic manure, rice straw, and inorganic fertilizer nitrogen(N) applications on soil total organic carbon(TOC)and SOC pools including water-extractable organic C(WEOC), hot water-soluble organic C(HWOC), potassium permanganateoxidizable organic C(KMnO 4-C), microbial biomass C(MBC), mineralizable organic C(Cmin), and the oxidizable fractions of decreasing oxidizability(easily-oxidizable, oxidizable, and weakly-oxidizable) were investigated in an 11-year field experiment under rice-wheat system. The field experiment included treatments of different combinations of farmyard manure, rice straw, and fertilizer N application rates, with C inputs estimated to be in the range from 23 to 127 Mg ha-1. After 11 years of experiment, WEOC,HWOC, and KMnO 4-C were 0.32%–0.50%, 2.2%–3.3%, and 15.0%–20.6% of TOC, respectively. The easily-oxidizable, oxidizable,and weakly-oxidizable fractions were 43%–57%, 22%–27%, and 10%–19% of TOC, respectively. The applications of farmyard manure and rice straw improved WEOC, HWOC, KMnO 4-C, easily-oxidizable fraction, Cmin, and MBC, though the rates of change varied considerably from-14% to 145% and-11% to 83% of TOC, respectively. At the C input levels between 29 and 78 Mg C ha-1during the 11-year period, the greatest increase was observed in WEOC and the minimum in KMnO 4-C. Water-extractable organic C exhibited a relatively greater sensitivity to management than TOC, suggesting that it may be used as a sensitive indicator of management-induced changes in soil organic matter under rice-wheat system. All the other labile SOC pools exhibited almost the same sensitivity to management as TOC. Most of the SOC pools investigated were positively correlated to each other though their amounts differed considerably. Long-term applications of farmyard manure and rice straw resulted in build-up of not only the labile but also the recalcitrant pool of SOC, emphasizing the need for continued application of organic amendments for permanence of the accrued C under the experimental conditions.     

Effect of conservation tillage on crop yield and soil organic carbon in Northeast China: A meta-analysis

Northeast China, the important grain-producing region in China, is under threat from soil degradation because of long-term conventional tillage (CT). The adoption of conservation tillage is anticipated to restore soil fertility, maintain crop yields and enhance sustainability. However, the integrated effects of conservation tillage practice on crop yields and soil organic carbon (SOC) remain unclear. In this meta-analysis of peer-reviewed studies conducted in the Northeast China region, we assess crop yields and SOC values under no-till, ridge tillage and subsoiling tillage practices. The results indicate that in areas with mean annual temperatures (MAT) below 3°C, crop yields were significantly (p < .05) higher under ridge tillage (0.8%) and subsoiling tillage (13.1%) compared with CT, whereas yields reduced under no-till (−3.7%). Ridge tillage generally had a similar effect on crop yield as no-till, without the negative impact in colder regions. We also report that no-till practice increased SOC concentrations by 24.1%, 43.9% and 17.4% in areas of higher temperature (MAT > 6°C), low mean annual precipitation (MAP) (<500 mm) and continuous cropping conditions, respectively. Ridge tillage and subsoiling tillage also had positive effects on SOC concentrations (to a lesser degree than no-till), indicating that conservation tillage can enhance SOC in Northeast China. Overall, the implementation of different conservation tillage measures in Northeast China was found to enhance crop yields and sequester carbon. We recommend that ridge tillage is used in colder areas and that subsoiling tillage is used in rotation with other tillage measures to maintain crop yields.     

长期施肥对红壤性水稻土团聚体稳定性及固碳特征的影响

施用有机肥是提高土壤有机碳(SOC)含量、促进土壤团聚体形成和改善土壤结构的重要措施。本研究旨在探讨长期作物残留和投入有机物料对水稻土团聚体分布及稳定性的影响,分析不同粒级团聚体的固碳特征及其与团聚体形成的相关性,以及土壤和不同粒级团聚体对累积碳投入的响应。长期定位施肥试验始于1986年,设不施肥(CK)、单施化肥(CF)、秸秆化肥混施(RS)、低量粪肥配施化肥(M1)和高量粪肥配施化肥(M2)5个处理。2009年采集0~10 cm土壤样品,测定总土以及大团聚体(LM,2 mm)、较大团聚体(SM,0.25~2 mm)、微团聚体(MA,0.25~0.053 mm)和黏粉粒(SC,0.053 mm)的质量比例及其SOC浓度,并分析闭蓄于SM内部的颗粒有机物(POM)、微团聚体(MA-SM)和黏粉粒(SC-SM)的质量含量和SOC浓度。结果表明,与CK和CF比较,有机肥混施化肥处理(RS、M1和M2)均显著提高了LM和SM的质量比例和平均当量直径(MWD),降低了SC质量含量;两个粪肥配施化肥处理(M1和M2)的效果优于秸秆化肥混施(RS),但是M1和M2间差异不显著;单施化肥则降低了稳定性团聚体的比例。团聚体的SOC浓度没有随粒级增大而增加,各处理均为LM和SM结合的SOC浓度最高,其次为SC,最小为MA。与CK比较,有机肥混施化肥处理均显著提高了各粒级团聚体的SOC浓度。总土SOC的增加主要取决于SM的SOC含量,而MA-SM组分决定了SM固持SOC的能力。总土、LM和SM的SOC含量以及从SM分离出的POM、MA-SM和SC-SM的SOC含量均与累积碳投入量呈显著正相关,但总土分离出的MA和SC的SOC含量对累积碳投入量反应不敏感,表现出碳饱和迹象。因此,尽管长期大量施用有机物料促进了红壤性水稻土大团聚体的形成和团聚体稳定性,增加了其SOC的固持,但有机质可能不是该土壤水稳性团聚体形成的最主要黏结剂。     

Estimating soil carbon dynamics in intercrop and sole crop agroecosystems using the Century model

Using process‐based models to predict changes in carbon (C) stocks enhances our knowledge on the long‐term dynamics of soil organic carbon (SOC) in various land management systems. The objective of this study was to apply the Century model to evaluate temporal SOC dynamics in two temperate intercrop systems [1:2 (one row of maize and two rows of soybeans); 2:3 intercrop (two rows of maize and three rows of soybean)] and in a maize and soybean sole crop. Upon initiation of intercropping, SOC increased by 47% after ≈ 100 years, whereas SOC in the maize sole crop increased by 21% and 2% in the soybean sole crop. The quantity of crop residue input was sufficient to increase the active (turnover time of months to years) SOC fraction in the intercrops and the maize sole crop, but not in the soybean sole crop. The slow fraction, with a turnover time of 20 to 50 years, increased in all crop systems and was the major driver of SOC accumulation. A 3 to 15% loss of SOC from the passive fraction, with a turnover time of 400 to 2000 years, in all crop systems showed the long‐term impact of land‐use conversion from historically undisturbed native grasslands to intensive agricultural production systems. This study provided an example of the potential of process‐based models like Century to illustrate possible effects of cereal–legume intercropping on SOC dynamics and that the model was able to predict SOC stocks within –7 to +4% of measured values. We conclude, however that further fine‐tuning of the model for application to cereal–legume intercrop systems is required in order to strengthen the relationship between measured and simulated values.     

Tillage effect on organic carbon in a purple paddy soil

The distribution and storage of soil organic carbon （SOC） based on a long-term experiment with various tillage systems were studied in a paddy soil derived from purple soil in Chongqing, China. Organic carbon storage in the 0-20 and 0-40 cm soil layers under different tillage systems were in an order： ridge tillage with rice-rape rotation （RT-rr） 〉 conventional tillage with rice only （CT-r） 〉 ridge tillage with rice only （RT-r） 〉 conventional tillage with rice-rape rotation （CT-rr）. The RT-rr system had significantly higher levels of soil organic carbon in the 0-40 cm topsoil, while the proportion of the total remaining organic carbon in the total soil organic carbon in the 0-10 cm layer was greatest in the RT-rr system. This was the reason why the RT-rr system enhanced soil organic carbon storage. These showed that tillage system type was crucial for carbon storage. Carbon levels in soil humus and crop-yield results showed that the RT-rr system enhanced soil fertility and crop productivity. Adoption of this tillage system would be beneficial both for environmental protection and economic development.     

Microaggregate-associated carbon as a diagnostic fraction for management-induced changes in soil organic carbon in two Oxisols

Carbon stabilization by macroaggregate-occluded microaggregates (Mm) has been proposed as a principal mechanism for long-term soil organic carbon (SOC) sequestration in temperate alternative agricultural and (af)forested systems. The aim of this study was to evaluate the importance of the Mm fraction for long-term C stabilization in Oxisols and to validate its diagnostic properties for total SOC changes upon changes in land use. Soil samples were taken from the 0-5 and 5-20 cm soil layers of native forest vegetation (NV), conventional tillage (CT) and no-tillage (NT) systems at an experimental site near Passo Fundo and one near Londrina in Southern Brazil. After aggregate-size separations by wet-sieving, macroaggregate-occluded water-stable microaggregates (53-250 μm) (Mm) were isolated from large (>2000 μm) and small (>250 μm) macroaggregates. Particulate organic matter located inside the Mm (intra-Mm-POM) and the mineral fraction (< 53 μm) associated with the Mm (mineral-Mm) were separated from the POM fraction located outside the Mm (inter-Mm-POM) by density flotation followed by mechanical dispersion. Sand-free Mm-C concentrations on a macroaggregate basis were generally greater under NV and NT compared to CT in the 0-5 cm depth at both sites. Our findings support the importance of Mm (especially the mineral-Mm fraction) as long-term C-stabilization sites in highly weathered tropical soils under sustainable agricultural and natural systems. At both sites, significant differences in total SOC stocks (g C m⁻²) among different land use systems were always accompanied by parallel Mm-C stock differences. Though total SOC did not differ among land use systems in the 0-20 cm depth at both sites, Mm-C stocks were greater under NT compared to the CT treatment in the 0-20 cm depth at the Londrina site. We concluded that in these highly weathered tropical soils the Mm-C fraction is a more responsive fraction to management changes than total SOC and represents a diagnostic fraction for present as well as potential total SOC changes upon land-use change.     

Effect of labile and recalcitrant carbon on heterotrophic nitrification in a subtropical forest soil

Carbon (C) is an important factor controlling heterotrophic nitrification in soil, but the effect of individual C components (e.g., labile and recalcitrant C) is largely unclear. We carried out a C amendment experiment in which either labile C (glucose) or a recalcitrant C (cellulose and biochar) was added to a subtropical forest soil. A ¹⁵N-, ¹³C-tracing and MiSeq sequencing study was performed to investigate soil gross heterotrophic nitrification rates, carbon utilization for soil respiration and microbial biomass production and microbial composition, respectively. After 2 days, results showed a significant increase of gross heterotrophic nitrification rate in glucose (GLU) (on average 3.34 mg N kg⁻¹ day⁻¹), cellulose (CEL) (on average 0.21 mg N kg⁻¹ day⁻¹) and biochar (BIO) (on average 0.13 mg N kg⁻¹ day⁻¹) amendment in comparison with the unamended soil (CK) (on average 0.01 mg N kg⁻¹ day⁻¹; p < 0.05). The contribution of heterotrophic nitrification to total soil nitrification was significantly larger in GLU (average 85.86%), CEL (average 98.52%) and BIO (average 81.25%) treatments compared with CK (average 33.33%; p < 0.01). After 2-month amendment, the gross rates remarkably decreased in GLU (average 0.02 mg N kg⁻¹ day⁻¹), and the contribution to total nitrification (average 8.73%) were significantly lower than that in CK (p < 0.05). A decrease in the proportion of heterotrophic nitrification to total nitrification in soil was also observed in CEL (average 38.40%) and BIO (6.74%) treatments. Nevertheless, BIO amendment (compared to CK, GLU and CEL) showed the highest gross heterotrophic nitrification rate, accompanied by a notably higher abundance of specific heterotrophic nitrifiers, i.e. Trichoderma, Aspergillus and Penicillium. These results point to a stimulatory effect of C addition on soil heterotrophic nitrification in the short term, while the stimulatory impact of C amendment diminishes with the decline in easily available C. In addition, a shift of the microbial composition in the long term can possibly be sustained for longer if additional recalcitrant C is available to heterotrophic nitrifiers. The dynamic response of heterotrophic nitrification to labile and recalcitrant C in this study offered an explanation for the positive effect of plantation and plant root exudation on the process.     

Intensive organic vegetable production increases soil organic carbon but with a lower carbon conversion efficiency than integrated management

Intensive vegetable production in greenhouses has rapidly expanded in China since the 1990s and increased to 1.3 million ha of farmland by 2016, which is the highest in the world. We conducted an 11‐year greenhouse vegetable production experiment from 2002 to 2013 to observe soil organic carbon (SOC) dynamics under three management systems, i.e., conventional (CON), integrated (ING), and intensive organic (ORG) farming. Soil samples (0–20 and 20–40 cm depth) were collected in 2002 and 2013 and separated into four particle‐size fractions, i.e., coarse sand (> 250 µm), fine sand (250–53 µm), silt (53–2 µm), and clay (< 2 µm). The SOC contents and δ¹³C values of the whole soil and the four particle‐size fractions were analyzed. After 11 years of vegetable farming, ORG and ING significantly increased SOC stocks (0–20 cm) by 4008 ± 36.6 and 2880 ± 365 kg C ha^?1 y^?1, respectively, 8.1‐ and 5.8‐times that of CON (494 ± 42.6 kg C ha^?1 y^?1). The SOC stock increase in ORG at 20–40 cm depth was 245 ± 66.4 kg C ha^?1 y^?1, significantly higher than in ING (66 ± 13.4 kg C ha^?1 y^?1) and CON (109 ± 44.8 kg C ha^?1 y^?1). Analyses of ¹³C revealed a significant increase in newly produced SOC in both soil layers in ORG. However, the carbon conversion efficiency (CE: increased organic carbon in soil divided by organic carbon input) was lower in ORG (14.4%–21.7%) than in ING (18.2%–27.4%). Among the four particle‐sizes in the 0–20 cm layer, the silt fraction exhibited the largest proportion of increase in SOC content (57.8% and 55.4% of the SOC increase in ORG and ING, respectively). A similar trend was detected in the 20–40 cm soil layer. Over all, intensive organic (ORG) vegetable production increases soil organic carbon but with a lower carbon conversion efficiency than integrated (ING) management.     

稻草及其制备的生物质炭对土壤团聚体有机碳的影响

向土壤中添加生物质炭已被认为是改善土壤质量,增加碳吸存的有效措施。通过模拟实验,利用同位素δ13C标记技术,研究稻草及其制备的生物质炭添加对土壤团聚体有机碳的影响。结果表明：稻草和生物质炭对土壤团聚体中新形成碳和原有机碳的影响截然不同。培养112 d,来自稻草或生物质炭的新碳主要进入到中团聚体（50 ~ 250 μm）中,比例为70.3% ~ 75.3%。与对照土壤相比,稻草添加显著促进了大团聚体（250 ~ 2 000 μm）原有机碳的分解（p <0.05）,但对中团聚体和微团聚体（<50 μm）原有机碳的影响并不明显,而生物质炭添加（SB250和SB350）则对大团聚体和中团聚体原有机碳没有显著影响,但SB250处理（土壤中加入250℃热解制备的生物质炭）显著抑制了微团聚体原有机碳的分解（p <0.05）,而SB350处理（土壤中加入350℃热解制备的生物质炭）的则无影响。对于同一粒级团聚体,稻草与生物质炭处理的区别,主要体现在新碳分配上,而对原有机碳的影响并不显著。